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Motor Control - 3 Position Start-Stop

Discussion in 'Electronic Design' started by alokw, Mar 4, 2007.

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  1. alokw

    alokw Guest

    Hi all!

    I'm working on a small project which requires an automated disc to
    rotate and stop in 3 positions.

    Basically, I have a small 8" disc made of foamcore, which I need to
    attach a small motor to. I am familiar with DC wiring and have used
    motors like this before, but have never had the need to do anything
    more complicated than reversing the polarity.

    What I'd like to do, is have some kind of controller, ideally, with
    three buttons. When I push button 1, the disc rotates to 120 degrees
    from its home position. When I push button 2, the disc should rotate
    to 240 degrees from its home position. When I push button 3, the disc
    must rotate back to its home position.

    Obviously, since the disc is only 8" in diameter, the mechanism will
    have to be relatively small. I'm also on a low budget, so whatever I
    can do to save money will be useful.

    Any ideas or suggestions will be greatly appreciated, as I'm not sure
    where to go from here.

    Thanks everyone!
    -Alok
     
  2. Sounds like you could use a servo motor from a radio controlled plane or
    car. They respond to pulsed signal. By feeding pulses of varying widths to
    the servo, the servo can be made to turn to a certain position and stop.
    These pulses fall in the range of 1mS to 2mS with 1mS causing the servo to
    move all the way in one direction and 2mS moving it to the other limit.
    Widths in between will make move the servo to a proportionate location.

    All you have to do is be able to generate 3 different pulse widths and
    switch between them. If you search for 555 and "servo tester" using Google,
    you will find sample circuits to drive servo motors. Here is one:
    http://www.uoguelph.ca/~antoon/gadgets/servo3.htm

    If you want to "use a PIC" instead, just say so. ;-)
     
  3. alokw

    alokw Guest

    Hi Anthony,

    Thanks for your quick reply!
    I did some research from the information that you sent me and had a
    few questions. These are mostly due to my lack of knowledge with
    electronic components.

    I was able to find most of the parts, but couldn't find any 390nF or
    100nF capactiors. I was able to find 390uF and 100uF, but I assume
    there is a difference between the two. Also, the diagram specs a "2K7"
    resistor and potentiometer. Is this short for something? I couldn't
    find anything with those labels.

    Is there a really big "amazon.com" of electronic components that I
    don't know about? Where do you usually get your components?

    Also, I wouldn't be opposed to using a PIC, but I wouldn't know where
    to start. Are there any great benefits to using a PIC?

    Thanks so much for your help!
    -Alok
     
  4. Jamie

    Jamie Guest

    use a stepping motor, other wise, you'll need to use something like a
    little DC motor that reverse it's direction via a little switching
    controller that monitors a set of photo detectors so that it knows where
    it is.
    of course, the motor will have to run slow so not to do swing loads
    which would cause oscillation..
    You could also look into using a Gray code decoder that can connect
    to the shaft. this reports position how ever, you then would need to
    build a decoder in something like a pic or avr chip.
    from my stand point that would be simple. from yours i don't know?
    Even using the stepping motor, you still need ways to detect the number
    of pulses. etc..
    i did how ever see one trick doing something like that. they drilled a
    hole in the shaft with 3 photo detectors on one side mounted in a
    concave pattern. the emitter on the other side of the hole. what it did
    was move the shaft slowly and when the emitter was passing through the
    hole, different detectors would pick up the intensity to stop or slow
    down the motor and even attempt to reverse it.
    I suppose you could put a set up like that on there.
    or you could simply use micro switches on a cam that is attached to the
    shaft one lobe on it. you could simply push the button long enough to
    leave one switch and thus the switch would then close keeping the motor
    running until it hit the next switch.
    I suppose you could use a 555 timer to do that.
     
  5. Jamie

    Jamie Guest

    Ah, that reminds me. you could use a Voltage Comparator and have a
    continuos potentiometer on the shaft.
    You just need three different voltage levels to select from. the
    output of the comparator would drive the motor one way or the other
    to balance the position of the disc.
    Using 2 comparators in a bridge mode to drive the motor.
    actually, 2 op-amps driving a dc bridge to position the motor will help.
    Just position the pot so that the cross over point is is between one of
    your selections.
     
  6. John Barrett

    John Barrett Guest

    radio shack sells an assortment package of "nF" valued caps (if you can find
    a radioshack that still sells components), and if you check some place like
    mouser.com or digikey.com, they should have what you need also. (or somplace
    equivalent in your part of the world :) Check around and see of you can find
    an electronics surplus store in your area.

    2K7 is short for 2.7 K or 2700 ohms

    a PIC or AVR microcontroller can do the pulse generation for you, and there
    is sample code available for controlling both R/C servos and stepper motors.
    The disadvantage is that you need to learn how to write code for the chips
    because of your custom application, and that you need a "programmer" module
    to load the compiled code into the chip, on top of the hardware that will
    actually hook the controller to the servo or stepper.

    The R/C servo approach has the advantage over the stepper in that you dont
    have to set up an "index" sensor to identify the "home" position -- thats
    already built into the servo.

    If you go with the 555 timer with switched resistors to control the pulse
    width for an R/C servo, thats pretty much going to be the simplest solution
    over all.

    Note there are 2 basic types of R/C servos out there -- "Standard" and
    "Continuous Rotation" -- you want the standard type -- CR servos are
    basically motors where the servo pulse controls the speed and direction, not
    the absolute position.
     
  7. Hello, please don't top post.
    Yes there is a difference of several orders of magnitude. :) 390nF is
    equivalent to .39uF, it's nano-Farads vs. micro-Farads, three decimal
    places. The 2k7 is another way of saying 2.7k Ohms of resistance. It's
    supposed to remove the confusion, ha ha. ;-)
    www.mouser.com
    www.digikey.com
    www.jameco.com
    www.sparkfun.com <- really cool stuff for the amateur mad scientist
    www.piclist.com is a good resource for that, but you may want to stick with
    the basics a little longer first. As to the benefits, they are the
    difference between putting a computer to work on a problem vs. doing it some
    other way. IOW, they can be astounding or mundane, it depends upon the task
    being performed.

    DISCLAIMER: They do require an investment in time in order to learn how to
    use them, but this is mostly a one time issue. You also need special
    equipment (though it's not that terribly expensive) to be able to flash
    (store) your programs into them. If you already know how to program, things
    will be much easier to learn.
    /DISCLAMER

    Perhaps a good way to get started in microcontrollers is to get something
    like a "BASIC Stamp" kit. You won't need any additional special equipment
    to program them and you do that using BASIC. They have nice tutorials to
    help beginners. You will find tons of information on that here:
    http://www.parallax.com/index.asp
     
  8. Can it always rotate in the same direction?
     
  9. Donald

    Donald Guest

    The speed of rotation would also be useful.
     
  10. maxfoo

    maxfoo Guest

    Surely you must have some old floppy disc, CD, VCR or whatever Drive collecting
    dust somewhere. Cannibalize the motor and associated circuitry to do your little
    project.
     
  11. I think I'd do it with relays and microswitches.

    Make a cam or bump on the disk, and mount three microswitches so that
    the cam will hit a switch at each desired position.

    When you press button 1, relay 1 operates, and latches itself through
    microswitch 1 and applies power to the motor. When the disk is turned
    so that the cam hits microswitch 1, that switch opens, and unlatches
    the relay, stopping the motor. Depending on disk speed, inertia, and
    friction, a brake may be required to prevent the motor from coasting
    too far when the power is removed.


    --
    Peter Bennett, VE7CEI
    peterbb4 (at) interchange.ubc.ca
    new newsgroup users info : http://vancouver-webpages.com/nnq
    GPS and NMEA info: http://vancouver-webpages.com/peter
    Vancouver Power Squadron: http://vancouver.powersquadron.ca
     
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